The present invention relates to a color image forming apparatus of an electrophotography type, and in particular to a color image forming apparatus which is small in size, highly maintainable and is excellent in clearing jams.
In an electrophotographic image forming apparatus that forms a full-color image or a monochromatic image, there are arranged around a photoreceptor drum, a charging unit that charges the photoreceptor drum, an image-wise exposure means by which the photoreceptor drum is exposed to an image, a developing unit by which a latent image on the photoreceptor drum is developed with toners, an image-transfer unit that transfers a toner image on the photoreceptor drum to a recording paper and others.
Among the aforesaid items, the developing unit in an electrophotographic image forming apparatus for full-color images is composed of three or four developing units each containing yellow (Y), magenta (M), cyan (C) and occasionally even black (BK) toners respectively. While the photoreceptor drum makes three turns, a yellow (Y) toner image, a magenta (M) toner image, a cyan (C) toner image and occasionally a black (BK) toner image too are formed on the photoreceptor drum and those images are transferred onto a recording paper, thus a full-color image is obtained. When an image forming system of this type is used, a color image forming apparatus can be made compact by making each of three or four developing units to be of a thin type and by arranging them closely each other.
In Japanese Patent O.P.I. Publication No. 55661/1986, there is suggested a color image forming apparatus of a type of image forming mentioned above wherein a latent image is formed by means of three laser beams corresponding to three developing units, and a photoreceptor drum, developing units and a cleaning means are united to be one unit which can be taken out of an apparatus main body. Further, the color image forming apparatus described in Japanese Patent O.P.I. Publication No. 55661/1986 can contribute to the improvement in terms of maintenance, but it has a disadvantageous point that developing agents tend to spill or mix with other developers when the unit is mounted or dismounted.
Further, Japanese Patent O.P.I. Publication Nos. 72159/1983, 43170/1988 and 431711/1988 disclose an example of a color image forming apparatus wherein a toner image of each color is transferred to a recording paper by means of a transfer drum, and a photoreceptor drum and its surroundings are united to be a unit.
However, following points are enumerated for the color image forming apparatus disclosed in Japanese Patent O.P.I. Publication Nos. 72159/1983, 43170/1988 and 43171/1988.
(1) An image forming unit includes not only a photoreceptor drum and its surroundings but also a transfer drum. PA1 (2) Both peripheral speed and the gap need to be kept in high accuracy between the photoreceptor drum and the transfer drum. PA1 (3) A releasing mechanism needs to be provided for releasing an engagement between the photoreceptor and the transfer drum.
As stated above, a color image forming apparatus employing a transfer drum tends to be complicated in structure as shown in above Items (2) and (3), which causes it to be expensive. In addition to this, it has been impossible to obtain fully a merit of a process unit of an image forming section because image forming means are not arranged closely around the photoreceptor, which is different from a monochromatic image forming apparatus that does not need a transfer drum.
The primary object of the invention is to offer a color image forming apparatus wherein easy maintenance for the apparatus is achieved by consolidating a developing unit, an image carrier and a cleaning means together or by causing each of the combination of two of them and other remaining one to be a cartridge capable of being loaded or unloaded independently, developers are prevented in a structure from spilling out or mixing with other developers when units, especially a developing unit is inserted or drawn out by consolidating a plurality of developing units into one unit, and easy clearing of jammed recording paper is achieved in the case when a recording paper is transported defectively.
The secondary object of the invention is to offer a color image forming apparatus employing the process in which a synthetic toner image such as a color toner image composed of toners of plural colors is formed on the surface of a photoreceptor drum to be transferred onto an image-transfer paper in one transfer operation, wherein image forming means such as a plurality of developing units and cleaning units are arranged closely each other around the photoreceptor drum in order to make the apparatus small and compact, and especially a plurality of developing units each is thin in shape are arranged at one side on the circumference of the photoreceptor drum, and upper framework is arranged so that it may be opened in the direction which makes an opening operation easier to offset the imbalance that the portion of a developing unit composed of plural developing units is heavier than the other portion.
The invention further relates to a color image forming apparatus wherein the apparatus is divided vertically into two parts so that the transport plane for a recording paper may be revealed.
There has been suggested a color image forming apparatus of a clamshell type wherein the apparatus is divided vertically into two frameworks and upper framework can swing for opening and closing for the purpose of easy jam clearing for recording papers in transport as well as of easy maintenance.
In the apparatus mentioned above, units and parts related to an optical system for exposure and to an image forming section are provided in the upper framework, while units and parts related to sheet-feeding and sheet-ejecting are provided in the lower framework, which enables the transport plane for a recording paper to be releaved widely when the upper framework is opened.
On such an apparatus, therefore, it is possible to clear easily the jammed recording paper by opening the upper framework, but it is necessary to remove the units and parts forming the image forming section such as a photoreceptor drum, developing units and cleaning units from the upper framework for each maintenance.
Therefore, for the purpose of handling the image forming section as a one group and of assuring the positional accuracy of units and parts of image forming section, they are usually arranged in one cartridge as a process unit and the cartridge is mounted detachably on the upper framework.
However, it is not easy to attach or detach the units and parts from the upper framework in a opened position without touching other units and parts because the process unit is large in size and heavy in weight, and further it is necessary to pay attention to the positional accuracy for the units and parts after they are mounted again.
Overcoming and improving the problems mentioned above, the tertiary object of the invention is to offer a color image forming apparatus wherein a process unit can be mounted on or dismounted from the upper framework safely and surely through a simple operation.